Basic Auth Support, Message Limit

Thanks to @danisawesome we have the following features now:

• Basic authentication via the mailHogAuth Cypress env var. This takes a JSON object with user and pass keys along with their respective values. Example can be found in the README (more info on auth handling here: https://github.com/request/request#http-authentication)
• An optional limit argument for mhGetAllMails(), mhGetMailsBySubject(), mhGetMailsByRecipient(), and mhGetMailsBySender() mail collection commands.